Air force planes on Tuesday and Wednesday spotted items in the water, including a seat from a plane and a seven-metre (23-foot) chunk of what looked like fuselage, that Nelson Jobim, the Brazilian defence minister, said were from the French jet.

On Friday Airbus reissued emergency guidelines to pilots of all the company's airliners after crash investigators said false speed measurements might have been behind the disaster.

French air safety investigators said automatic messages broadcast by the Air France jet just before it plunged into the Atlantic on Monday had shown the plane's systems were giving different readings.

Kieran Daly, editor of internet journal Air Transport Intelligence, told Al Jazeera the failure of the plane's ability to measure speed would make it "incredibly difficult to fly".

"It was very challenging circumstances trying to fly the aircraft in what was already as we know severe turbulence in this thunderstorm.

"It really would be extremely difficult to retain control of the aircraft. It's difficult to think of many worse situations that the crew could find themselves in. They may have only had a few seconds or few minutes to react... and it is possible that even a really highly capable crew may not be able to work out what was happening quickly enough to save the aircraft."

The Air France plane came down early on Monday as it was transporting 228 people from Rio de Janeiro to Paris.

Speculation over what caused the accident has ranged from a massive, lightning-packed storm in the area at the time, to turbulence, to pilot error or a combination of factors.

No mayday call was received from the plane, just a series of data transmissions signalling it had lost power and then had either broken up or gone into a fatal dive.

'White flash'

Memorial services were held in Paris and Rio for those on board the plane, though no bodies have been spotted at sea.

Many relatives of the passengers attended, but others declined, refusing to give up hope that somehow, despite the evidence, their loved ones had survived.

Bernard Kouchner, the French foreign minister, said after speaking at the Rio ceremony "it will probably take some time" before the reason for the catastrophe - the worst in Air France's history - would be known.

The point in the Atlantic where the plane came down is "immensely deep", between 3,000 and 4,000 metres, complicating the search for the black boxes, he said.

A Spanish pilot who was flying at high altitude some distance behind the doomed Air France jetliner said he saw an "intense flash of white light," according to his airline, Air Comet.

A co-pilot and passenger also saw the bright light, according to a report initially given to Spain's El Mundo newspaper.

"Suddenly, we saw in the distance a strong and intense flash of white light, which followed a descending and vertical trajectory and which broke up in six seconds," the captain wrote.